I am new to making Sims 4 cc... Help!

I decided to make the leap into first artwork. Been using photopea to make it. Please be kind with me, I have a horrible learning disability and don't get how to do things at first. But last night after I did my thing on Sims 4 studio, I moved the package over to photopea it says "Unknown file format: DBPF" yes I have looked it up and right now it's just jibberish to me. Does anyone know what I can do? I don't know how to look up code and I did it just fine then something happened and idk what I did.