** UPDATED 6/4/2018 **
(I will continue to update this list as I find more challenges.)
I was recently looking to see if there were any new art challenges to try this summer, and stumbled upon the #kidlitsummers hashtag... It is a fun (but relaxed) challenge that "draws on" summertime memories from childhood. It runs from June 1st - August 31st, but there aren't any strict deadlines or rules... It's perfect for my unpredictable summer schedule!
Since I thought you might be looking for a challenge too, and because I thought it would be handy to have all of the art challenges (that I know of) in one place, I have created the following list:
(created by Lily Williams)
This Instagram and Twitter based challenge only has 3 "rules":
1. Draw your favorite summer memories from being a kid.
2. Post them online with the hashtag #kidlitsummers
3. Share images any day from June 1st - August 31st.
(created by Daisy Yellow)
"A creative challenge, not a competition.
Make something on an index card every day for 61 days. That's it."
This challenge runs from June 1st - July 31st.
(You can see some of my past index cards here.)
(created by Rhianna Wurman @ellolovey)
An Instagram based challenge with daily drawing prompts.
This challenge runs year round (prompts are updated each month).
(created by Leah Piken Kolidas)
Creative Every Day is a relaxed year long challenge to stay on track being creative (in some way, any way) every day. There are check-ins and optional monthly themes. In November, the challenge is kicked up a notch with Art Every Day Month (AEDM). "The idea is to bring more creativity into your life, not to make you feel overwhelmed, pressured or guilt-stricken."
(IllustrationFriday.com)
A weekly illustration challenge with a new theme to illustrate each week.
Artist can submit illustrations to their online gallery.
(You can see some of my past Illustration Friday posts here.)
(@Clr_Collective)
A Twitter based challenge.
"A weekly challenge where you make work based on a single colour. Colours announced on Saturday; post your work Fri at 19:30 GMT."
(@AnimalAlphabets)
A Twitter based challenge.
"Weekly challenge based on a letter. Post together Mondays 19:30 GMT"
(A new alphabet is just starting - the theme is "extinct animals".)
(created by Fabi Santiago and Sheena Dempsey)
An Instagram and Twitter based challenge.
"2 themes = 1 illustration" (A mash-up of 2 randomly chosen themes.)
New themes are chosen weekly and are due on Wednesdays.
(created by anouk.adel.art)
An Instagram based challenge with a new theme and guest host each week. New topics are posted every Sunday.
(created by Bobby Chiu and Jim Bryson)
A weekly Instagram based challenge where topics are pulled from 2 jars:
1 adjective + 1 noun = 1 drawing
(They have a YouTube channel where they share their drawings every Thursday.)
(SCBWI.org)
The Society of Children's Book Writers and Illustrators monthly art prompt.
(Members have the opportunity to share their work in the online gallery.)
(created by #kidlitart: Rubin Pingk, Diandra Mae, Blythe Russo)
Created by the same people who host the weekly Thursday night #kidlitart chat on Twitter, BAPC is an annual art challenge with monthly prompts to help illustrators build their portfolios.
(Click here for the official rules and prompt list.)
(created by Sheena Monahan @3yroldscribble)
A monthly Twitter based challenge.
"A 3yr old's scribble posted on the 1st of the month! Finish it and tag with #3yroldscribble. Post on the last day!"
(created by Michael MacVean)
"A weekly art meetup to engage and encourage each other, exploring the same theme each week. Choose your preferred platform and join us there!"
(created by Beatrice Blue)
"Childhood Week is a 2 week event distributed in 2 separate weeks along the year: One for Summer, one for Winter. The main goal is to remember and show each other our childhood with either paintings, photographs, anything fun!"
(Click here for the official rules and prompt list.)
(created by Jake Parker)
"31 days, 31 drawings" (in INK)
(#mermay)
I'm not sure who was the first one to start this challenge... In May, artists devote the month to creating a new mermaid themed drawing each day (and labeling it with #mermay). There are always many prompt lists if you're looking for ideas.
And last, but certainly not least...
The weekly challenge that I helped create 7 years ago...
(created by EVA and Me!)
Paint Party Friday is a relaxed weekly challenge for painters (of all types) to check-in, share works in progress, and/or finished paintings.
These are all of the art challenges (that I know of).
Please let me know if I've missed any!
Happy Creating!!!
11/100: Sure, Dolores may not have technically *needed* a mask, but she was always a bit of an overachiever... and that definitely included masquerade ball accessorizing. 
12/100: Randy wondered if coming to the party might have been a mistake...
13/100: Phineas is ready to party, but transportation is turning out to be trickier than anticipated.
15/100: A sketchy start inspired by Bulldogs Are Beautiful Day.
16/100: When Steve goes to a party, there's usually a lot of swatting and shooing. But an Earth Day Party?!? He's a 
18, 19, and 20/100: A few feathered friends...
