I am so sorry that you may have lost some images/sketches. We have reports of broken images but I can't seem to reproduce the issue.
I would start making versions of the sketches note in step 7 here:
https://forums.autodesk.com/t5/sketchbook-forum/android-sketchbook-tips-and-tricks/td-p/9385407
Also, there are 2 x Gallery on Android. One is in SketchBook (/Autodesk/SketchBook3) and one is in the Android App. The "Gallery" in Android is for Photos/Screen Captures etc. If you delete a sketch there, it may delete a preview only so you can try to import any .tiff file in the /Autodesk/SketchBook3 folder.
It is noted in Step 9 in Tips and Tricks post. To be certain, I would backup all sketches from /Autodesk/SketchBook3 on Mac or Windows computer and open it in SketchBook (free) via www.sketchbook.com/thankyou
David Lau