Hi, I have posted before on this topic but I wanted to share DDs favorite books/series that are not too long and have colorful pictures (her requirements):
Cowgirl Kate & Cocoa - Erica Silverman (about 6 books in the series, I think)
Keeker - Hadley Higginson (about 8 books in the series)
Amelia Bedelia - lots of books
Minnie & Moo - Cazet (lots in the series)
Heny & Mudge - Rylant (lots in the series)
Annie & Snowball - Rylant (about 6 in the series)
Mercy Watson - diCamillo (about 5 in the series)
Mouse and Mole - Yee (a few books)

HTH and do report back if she liked any of these!