How Do I Add a Photo to a Recording?
You might not even have to! Slopes can automatically display photos you took while riding, without you having to "import" them or anything. Set the Display Photos setting to Taken During Recording and photos taken during your day will be pulled through into the Logbook. You can access this setting via Account > Settings > General.
Sometimes there is a photo that perfectly captures the day, and you can attach one to the recording directly (It'll always be the first in the slideshow, and it'll stick around even if you delete it from your photo library. same as how other fitness apps do it). To add a featured photo, open the recording that you'd like to add the photo to via the Logbook and tap Edit in the top right. You then just need to tap on 'Attach Your Favorite Photo', select a photo and tap Save.
If you've enabled the photo library integration above, you can also tap the "..." when viewing the photo full-screen in Slopes to see the attach option. Useful for old recordings where scrolling back a few years is a pain.