You can do the entire thing as a one-liner using only find:
find ./ -type f \( -iname "*.jpg" -or -iname "*.png" \) -exec sh -c 'mv "$0" "$(uuidgen -r).${0##*.}"' {} \;
Test on my machine:
phaedrus@sys76 ~/D/test> ls -lh
total 0
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test1.jpg
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test1.png
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test2.jpg
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test2.png
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test3.jpg
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 test3.png
phaedrus@sys76 ~/D/test> find ./ -type f \( -iname "*.jpg" -or -iname "*.png" \) -exec sh -c 'mv "$0" "$(uuidgen -r).${0##*.}"' {} \;
phaedrus@sys76 ~/D/test> ls -lh
total 0
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 062d8954-9921-42bd-ad24-0e4ed403a5db.jpg
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 111f859f-b1fe-4488-b2bc-75585320e3a3.png
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 39b9fe4e-7a05-43c9-b30a-69e9a13aa3a9.png
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 57bda91e-49e5-43fe-8318-aeeb2e3adde7.png
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 97398eb7-54aa-488f-8fbe-0b84b5e5a50d.jpg
-rw-r--r-- 1 phaedrus users 0 Dec 6 01:08 f7a13274-e2c0-4fa7-9907-c590d1280c2e.jpg
btw, Lemmy doesn’t like language specifiers in the multi-line code blocks, so it’s difficult to read all that in its current form since there are no tabs to know how you have it formatted. Makes it virtually impossible to troubleshoot your specific script.
edit: further reading on the ever useful variable expansions (${0##*.} portion of my one-liner):
https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
It might be instance related, I’m on PieFed, so perhaps the markdown implementation is different.
Also, I realized that the parameter expansion might not be straightforward and added the GNU docs on it, but looks like you found a post about it at the same time! Glad to hear it got you sorted out.