Keywords are the main thing. You dont need to find them, most often they find you. In TV niche yuor main keyword with be LED TV, TV, television etc. Pretty simple, right?
All of these need to be used not only in texts but in alt as well. As far as i know this is one of the most valuable thing when it comes to images in SEO.
Just dont forget all text MUST be readable whether its title image, alt description or similar.
My general advice is you have much spare time dealing with all those IMG alts and titles would be to describe every picture and put some LSI or longtail keyword (or a main keyword) where it belongs but not overoptimizes the page
All you can do on submitting image on SEO. The image must be related to your website in different image submission or sharing sites. Image submission involves choosing the right image, file format, alternate text, and image description.
You need to create the innovative image which should be related to your website an then post it to the different image submission sites. There are lots of image submission sites, you can search it through Google.