Bit sad that projectiles can't be changed. Will it be possible in the future?
If not, is the bubble gun possible to be changed in place of that?

Might be a bit blurry...
It's good but looks like you didn't read the instructions,
Or just instruction was too bad...
1. You have to make only one file for quote. This means you have to merge all drawing you had drawn in one. I recommend to use game maker sprite editor, or making it in .gif file.
2. You have to split item. I accept items only as a part. In your case, you have to split head part and body part. Please check Characters.zip and you can understand what I mean.
3. I strongly recommend to leave portrait's background as transparent color, and only draw item sprite for it unless you can make it better than I suggested. You might change your portrait, delete background color and if you're making head portrait, leave head part of items and erase quote's face and add more details or shade if you want then outline it with 1px white color(because it looks good). I'm recommending this way because it doesn't make people confused about what item it is.
Edit:
Yes bubble gun can be edited!