For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
If you want to do such stuff you must be familiar with php, php file handling etc. for taking the two images, create a new image(overriding the original one or creating a new one and modify the stored filename).utilise PHP GD in a PHP form plugin, onBeforeStore or onAfterProcess