I want to make my game moddable, is there a way to make the character images modifiable just like in friday night funkin?
You don’t make a game moddable the whole point of mods is someone modifying the game files directly. If they do it through an interface offered by the game it can be an extension, texture pack, add-on, dlc, you name it but not a mod since they are adding a package to the game not modifying the game to add/change that thing.
As of now there are no way to make such texture packs in GDevelop through events. Your players can modify the images and code (that last one requires coding knowledge though) by messing with the game files or using GDMod.
I just want to be able to change the textures after exporting the game, just like making a custom skin
In that case I’ve already answered your question:
Thank you for your help 
You can use filesystem like ‘load txt file from’ or ‘load json file from’ (didnt try that though)
That has got nothing not do with either modding or texture packs