The way to upload photos depend of the device you are using:
Mobile: When you tap on Photos, you can choose Google Photos as a source for your photos. The photos are downloaded, then uploaded in the post, with the limit of 3Mb size
Desktop: You can embed a photo stored in Google photos. In this way, the photo is not counting in your quota of 1000 photos, and there is no size limitation.
Embedding images directly from Google Photos is really bad idea, because such embeds just after few days may loose proper linking.
Photos are not a proper service for hosting images to the web. GPhotos is a service for private photos management not for hosting.
glad to see you here. I am sharing a procedure given by Googler. What I do for keeping the link is to create an album for every specific post, and I never modify it after posting