Saving will increase the minimum version of the IDE required to open this project.
All this is saying is that the IDE version will now come in line with the version of Xojo you are using.
Most projects are marked internally as allowing Xojo2007 to edit the project.
But once you add an image (the PNG you mentioned) it now requires that the IDE supports Imagesets , which 2007 did not.
Therefore, this is NOTHING to worry about, and in my opinion that message does nothing but cause undue concern on the part of the developer… I know it did for me for a while