I’m not really an expert on this stuff but I can tell right away that your image dimensions are probably too large.
20x25 inches at 300ppi means that you could print your image with those dimensions (ppi, or pixels per inch, is basically a translator tool between the digital and physical worlds, and has no meaning if you have no intensions of printing your work).
The thing you want to pay attention to in digital work is pixels. With your settings, the dimensions of the image are 6000x7500 pixels. The reason why your image appears kind of off on tumblr is because tumblr scales your image to fit the dashboard view (which is 500x750 px) automatically with silly computer logic that has no regard for image quality.
I suggest you scale your image down yourself with whatever software you’re using. I think the biggest dimensions tumblr can handle in a photo post without screwing up are 1280 px in width and 1920 px in height.
If you’re using Photoshop then the easiest way is by Edit > Image Size, switch your dimensions from inches to pixels and replace either the width or height with the tumblr limit so that both of them fit inside the limit (make sure you have Scale Styles, Constrain Proportions and Resample Image checked, and if you’re scaling down then maybe choose Bicubic Sharper from the sampling dropdown menu).
Oh, and don’t forget to save it as a different file! You should keep the high-res versions of all your work just in case you want to fix or print them later.
And if you’re worried about colors, you could check your color profiles (in PS Edit > Color Settings, or if you want to change the color profile of an existing image Edit > Assign Profile). For best color range on screen choose Adobe RGB.
Again, I’m not really the highest authority on this and if any of you guys have something to correct or add to this, I’m sure it’d be appreciated! (-:
trying to forget a horrible memory like