i. Open “Instant Upload” under your Google+ Photos
ii. Hit the End key. Your browser window will scroll down to the bottom of what’s currently displayed. Since Google+ will load up more photos, hit End again. Repeat as necessary.
iii. Click and drag from below and to the right of your photos to start selecting them, like you do when selecting multiple icons with the mouse on your desktop.
iv. Without releasing the mouse button hit the Home key to scroll all the way back up to the top. You should be able to fine-tune the select area to capture all of the photos.
v. Click the “Delete” button (only visible at the top of the page).