Click on SETTINGS. Look at the side. Scroll down to MY ACCOUNT and click on GENERAL SETTINGS. Scroll down until you see THREAD DISPLAY OPTIONS. In that section, make sure you have SHOW IMAGES checked. Also in that section look for 'THREAD DISPLAY MODE'. Is it set to set to 'Linear-oldest first'? If it's not, then click on the little blue square and find it and click it.
Scroll down to the bottom and click SAVE CHANGES.