I come from the time of making signatures of sports athletes in Photoshop. I used to get nice quality photos from a site named elibrary although it looks like it has changed. I am looking for a nice website that has large pictures of everything, whether it’s sports games, celebrities, stocks, landscapes, etc.
The landscapes you mentioned are easy enough to get from most any stock site, like iStock or Shutterstock.
Photos of sports figures, sports teams and celebrities are problematic, though, since they have a vested interest in managing their celebrity status and how other people use their fame to make money.
In other words, unless the photos are genuinely newsy or taken in public places where there’s no expectation of privacy, these kinds of famous people are sort of inclined to not give permission for their photos to be distributed and are equally inclined to file lawsuits over them. All this makes these kinds of photos a little difficult to obtain, and a little risky to use when they are found.
The US has laws regarding the Rights of Publicity. Your use of and selling of such is an infringement on that right.
But you say they are Sigs. Technically all sigs are copyright infringements but they fall into a weird area that is encompassed by the concept of “fandom.” There are some times when a celeb or franchise will turn a blind eye to things such as sigs, fan art, and fan stories in the interests of keeping the fan base happy. But once you turn commercial, or if a celeb objects to the usage, all bets are off. I once saw a TV star shut down all the photo sellers at a science fiction convention because people were bringing him bootleg photos to sign (ie non-approved still photos.) You get the drift.
There are any number of free sites out there for landscapes and stock photos. Celebs, not so much. I do know a few sites that will assist you at getting usage permission if the use is other than editorial, but it’s doubtful, for sigs, that you will want to pay what that costs. (Arm+leg+a lot of other body parts.)