Many internet sites which host different things such as photos, etc., will have some bandwidth limitations. This can be especially true for free sites which also have a paid membership area where you can get additional bandwidth.
This may be the case with your photos. Everytime someone loads a page where your post is, and which has the link to the photo, that photo is automatically downloaded to that person's computer. This takes processor time, and the amount of data downloaded (as bytes) is known as bandwidth. You may be allocated x-number of megabytes, or gigabytes of bandwidth. For example if you get 100Mb, and you have a photo that's 2Mb in size, then once it's been downloaded (viewed) by 50 people, it's at the 100Mb limit.
Many of these are on a monthly basis, so your photos may automatically appear again on July 1st. This would work until your bandwidth limit was reached again, and then the photos wouldn't show anymore. This appears to be the case for your photo.