If you're using the iPad version then any painting you saved should be listed in the gallery. If it's not there then unfortunately the only way to retrieve it would be to restore your iPad from an older backup (using iTunes) that contained the painting. Have you checked the List button (top right of the gallery) to see if the painting is listed there?
It's important to back up your painting files when using the iPad as anything that forces the removal of the app from the device, such as some operating system updates, will remove all of its custom files as well and that includes your paintings.
Matt
ArtRage UI
Ambient Design.