imma just pretend like i know what this means or how to do it
The error the game threw up is saying that it couldn't find the image file it was looking for. In this case, the image file does exist, but the name is different from what the code was looking for, so you can just rename that image file to fix it.
You need to open your healslut game folder and then navigate to the [healslutgame v. whatever]\game\images\hs\slime folder. Then, rename the "girl-classic-ohno.png" file to "girl-classic-ohh.png"
If you don't know how to parse a folder tree, each name between the "\" is the name of the folder opened, i.e. open the "game" folder and in there is the "images" folder, then the "hs" folder, etc.
Edit: To amend, as docjane112 stated, it would probably be best to copy the file and rename the copy rather than renaming the original file just in case the original name also gets called by code somewhere.