If the images are relevant then you're tossing earned SE traffic and extra value in the other ranking signals Google uses. If you have 1000 posts on gardening a

If the images are relevant then you're tossing earned SE traffic and extra value in the other ranking signals Google uses. If you have 1000 posts on gardening and an image for each with gardening in the filename, you're going to have an easier time ranking for gardening.