I recently bought a digital frame for some of my more elderly relatives.
The frame's firmware was quite retarded, and I had 2 issues with it:

1. Pictures were always displayed in order

This might be a "not a bug but a feature" type of situation, but still.
I solved this using bash's $RANDOM, which evaluates to a different random number every time:

rename 's/\.jpg$/.jpg/i' *.* # Just makes it look nice
for F in *.jpg; do mv "$F" ${RANDOM}.jpg; done

I couldn't use xargs rather than the for loop, because I needed to evaluate the variable for every file

2. Pictures were stretched to the frame's proportions

Assuming I treated my relatives with the following apple pictures:

The frame would show them like this:

Obviously, there was no setting in the software to stop doing that, so I had to modify the pictures themselves. I settled for resizing the pictures (without modifying the aspect ratio) to a certain resolution (the frame's screen's resolution), and padding it if it's not the right size (for instance, the picture is too short / too wide).
I modified the pictures to look like this:

The script uses ImageMagick, which is apparently built for these things (I initially considered using gimp).

RES=2048x1536
BG=black
ls *.jpg | xargs -n1 mogrify -gravity center -background $BG -resize $RES -extent $RES

Note I'm using mogrify, which performes in-place updates of the file. To create a different modified file, use convert and specify a new filename at the end.
Note2 resize does the actual image resizing (while keeping aspect ratio, unless specified otherwise), and extent fills in the missing sides with the background.

attribution

Delicious apples taken from: