Trouble adding new pics with the OOB editor

[BogdanM]

Ok so I've downloaded Ultimate Paint, edited a picture, saved it as lbm and then successfully imported it with the OOB editor. The only issue is that UP doesn't want to load the game palette. Is it really necessarily to change the palette? If it is, will it be ok if I edit the picture with PSP, save it as jpg then load it with UP and save it as lbm?

@Cross
If the answers to my questions are not positive, I will try the tool in the topic you mentioned tomorrow.

Thanks everyone for your help!

PS. I'm trying to replicate some of the missions from the Modern Warfare games, with new maps, weapons, characters. I'm still doing tests and trying to figure out how to get Soap, Price and Yuri get the upper hand against dozens of militias... This mission might be a bit too much but other levels are doable, I've already tried my mapping skills doing the Hamburg level and after a few test I'm feeling optimistic about this project. Anyway if I manage to do anything worth playing I will certainly share it with you guys!