Image SEO – What file size?

(…) and then a bottom (…) that will open in a new window (…)

Link quite normal so that the bottom is opened in the same browser tab. If a user wants to open the page in another tab, he can determine it using the context menu of the browser.

When it comes to showing photos enlarged again, you can Lightbox use.

(…) that the photos of file size should be relatively small max. to 200 kb.

It’s right that it’s good if your images tend to be small in file size and thus less time to load. However, search engines such as Google do not look at the size of individual files but consider the entire load.

(…) which I do not get to this size without seeing noticeable quality loss.

Some set screws you could look at:

• WebP compression, alternatively (if you don’t care about a support from Edge) AVIF would be even better in compression
• Use Sourcesets. This means that you can store different image variants for different screen resolutions. The browser can then decide which image variant it chooses.
• Media Caching via CDN server (e.g. CloudFlare, Fastly, …)
• Lazy-Loading mechanisms

My question is now how relevant that picture-SEO is at all for the subpage / photo gallery (…)

They can be treated separately.

(…) and the images there may be all in the range 1-2 mb?

However, you should not place actual user evaluation under a ranking on a search engine. SEO should be used to make your website more user-friendly.

If a user notices that a charging process takes a lot of time (and you still have to calculate that not everyone has a good Internet connection), then the probability is great that it is gone again within a few seconds. Maybe in time he saw a picture, but picture 2 and 3 are no longer of interest.

See that your images remain below 1 MB as much as possible and your website generally does not have long loading times (PageSpeed Insights can help).