For a variety of reasons stacks ignores the file name of the original image file that you drag in. If you’d like to to maximize SEO you should add a descriptive name to the Filename field you mention. This name will be used for publishing. If you don’t set the name then Stacks will use a generic name.
In some cases, when there is more than one image uploaded (for example when a retina resolution image is also published) then the name is used as the base-name and resolution info or size is appended.
Setting names is good for SEO – but how good is black magic, Google only provides general recommendations and their recommendations change often.
In my experience naming the prominent image(s) that are part of relevant to the search term you’re optimizing is the best. Other images scattered around every web page (like buttons and borders and things) have no impact at all.
About file sizes:
Stacks works hard to “do the right thing” with images. But there are a lot of cases and details here. I could write several pages about it. But I’ll try to stick to just resolution and file size…
When you drag an image into Stacks it will store a copy of the original image in your project. This is the full resolution uncompressed image. Whenever you make changes in the image editing window those edits are performed on the original image – so maximum quality is always preserved no matter how many edits you make.
Changing the Max Width and Max Height dimensions to values smaller than the original – or the Scale setting – will result in a smaller exported image.
If you scale the image to less than 50% of the size of the original in both dimensions, then Stacks will also export an @2x image the the correct HTML 5 image tag to show retina resolution images on devices that can see them.
Gif images are a special. Since they often have highly optimized color pallets and potentially animations with reversing etc – they can pose many problems in trying to scale and re-compress. In those cases Stacks disables all of the editing controls – although you can still change the filename and alt tag, of course.
There’s 10,000 more things I could say about images. But I think that’s enough for one post. If you have any other specific questions – or want to make a suggestion about a future upgrade, don’t hesitate.