SB-Collage

hello, im new and inexperienced at php so forgive me if this is a atupid question.
In the Jakarta theme, the main picture ‘sb-collage’ is too big for me. I’ve tried resizing the picture in adobe photo shop but when uploaded, there’s just plain yellow colour where the deleted part of the picture would go.

i want to know what file i should edit (and if you can tell me what part of it) so that when i resize my picture, the yellow space is gone and my posts are lifted up.

sorry if no one understood that

appreciate some help

spideee