Yellow stripe on screen saver

That looks like a photo of the watch, which shows the problem. The next thing to try is capturing it as a screenshot How to take a screenshot on Apple Watch - Apple Support so you have an indication whether it is a fault in software and screen rendering (the problem still appears) OR it is a hardware defect in the screen display panel (the problem never appears in a screenshot). If the defect doesn’t show in a screenshot you might need several attempts before you are sure you didn’t just miss the moment.

Also please try to notice if it appears with more than one photo, and if you view a known problem photo as a photo rather than as your screen wallpaper (rule out something in the software to display photo wallpaper)

You might try a reset by pressing in the crown and side button until you see the Apple logo (about 19 seconds), then releasing both buttons. Then see if it still happens.

Can you post a screen shot?

Does it appear if you just open that picture by itself?

That was a typo...I meant Approximately 10 seconds. Good luck.