This is admittedly a rather unexpected ask, and probably murkier waters than I'm anticipating, but I love Stardew Valley so much that I'm inclined to feel happy about this! I adore the idea of seeing my own work in the game somehow, so you have my permission to give this project a go.
The only thing I can think of, in addition to what you've already mentioned about credits and a URL link to this page, is that you also include in the terms & conditions of your mod something along the lines of "for personal use only; further modification and/or redistribution of the graphics within this mod is not allowed under any circumstance."
(Note to others that might find these comments: I'm only addressing and permitting @SHISIQUE in this particular case; anybody else with a similar idea, interest, or inquiry should ask me personally for permission in a separate comment somewhere on this page.)