Well it might not seem apparent but onsite SEO, as well, includes multimedia, narrowly, pictures. If you have pictures of cats when you are showing a topic like boats it is a stretch in credibility and relativity. In the background to most multimedia are metadata, that, in fact, describe the content, for the Search Engine spiders that do the “cruising” of web sites to take inventory to assess. If for instance there is no meta data then the item will do you no benefit. If it does and is not relative it may do damage but certainly will do no good to the purpose of having a web site . When properly defined and of proper relevance it will add to the weight of relative content and in it’s diversity will give you an advantage among your competitors who do not follow these guidelines.
Now it is not clear if there is an issue with size of the pictures. In my experience a balance between size and fidelity is required since part of the analysis of your website is load time. Smaller more consolidated resources that are perhaps less good quality wise than is available may ultimately be an acceptable trade-off for site performance. It is common practice to consider small pictures icon’s. So a banner or logo that serves it’s purpose as a banner or logo when shrunk down becomes an icon or favicon. One interesting point is it is in some instances a plus when you have a favicon when there is a placeholder for one. So as a rule if you can place one in a set space set aside for a favicon or icon you will only benefit for inserting one. Now with the advance of interface languages and programming efficiency it is now easy to even add a motion favicon.