These are some simple tips you can use to make sure your images look consistent and are not slowing down your site. Most importantly, you are getting good indexing with Google so you can be found!
1. Decide what size is best for your post, image gallery, thumbnails, and the like. Make your images twice this size. So, 1200px by 800px would be 2200px by 1600px and keep the DPI at 75. This will do two things for you. First of all, it makes your computer faster by not having to resize the images. Secondly, you will have the right size images for retina display.
2. Set your image sizes (use the dimensions from step 1) to keep your images consistent and save time editing. Go to settings/media in your sidebar and set your image sizes here. Now, when you upload an image you can choose from your preset sizes and WordPress does the editing for you.
3. Have you ever Googled something and clicked on images? If you want your work to index here, just add searchable terms to your images. So, instead of “fine art print 1” it would be “Mid Century Modern Digital Art Print.”