Were these items related to any quests you'd recently done? Usually this happens when the client doesn't realize that an item is (still) there, but in cases like that you don't see anything when you hover over the item, either. If you can help me narrow down what caused the item icons to go blank then we can try to track this down.
I had this happen to me after I cooked something at one point. I didn't write down what food it was, although I think I made it with the frying pan. The food had an image in the menu, but once it was made and put in my generic bag, it didn't have an image. I only noticed the alt-text when I couldn't put something in what I thought was an empty slot. I solved the problem by eating the food!
I believe I saw the missing icons the first time I opened the bag after logging into the new testing session of the game. I believe the items were all fruit items made with the fruit changing machine during the previous test session. Sorry I can't be more precise.
To be clear, the missing icons did appear on their own after I refreshed the game in my browser.