Let's assume you're logged in as [webname_one], and you have the permission to create and edit cards on /cards/[webname_two]. When you want to embed a photo, you can do it only with photos which are in your own photo collection /photos/[webname_one], but not with photos which are in /photos/[webname_two].

So either you stop posting cards on /cards/[webname_two] or you upload the photo also on your own photo collection /photos/[webname_one] before embedding it into a card on /cards/[webname_two].

Any document (also photos) can also be uploaded by using the upload button on /cloud/[webname] instead of /photos/[webname], but this, I don't recomend for photos, because then, the photos don't have a permanent link.

When uploading a photo by using the upload button on /photos/[webname], then an (invisible) permanent link to that photo is created automatically, and that permantent link will be used by the system when embedding the photo into a post. This is very useful because the photo will always be found and shown even if you move it from one photo album to another. Another good thing is that photos uploaded like this are shown in /gallery/[webname].

Another channel [webname_two] can give you the permission to upload photos like this onto his /photos/[webname_two].