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Typically, images up to 13 inches wide are displayed without causing the horizontal scroll bar to appear when the browser is full-screen. Wider images make scrolling necessary and may cause the page, the wiki markup, or both, to be displayed improperly so that lines no longer wrap.
 
You can attach images at any size and change the display size in Confluence. Thus, you can use the original image capture which provides the best resolution with the least loss of data. Try specifying the width only, in even increments calculated based on 72 pixels per inch:
Image Width in Inches = 13, Width in Pixels = 936
Image Width in Inches = 12, Width in Pixels = 864
Image Width in Inches = 11, Width in Pixels = 792
Image Width in Inches = 10, Width in Pixels = 720
Image Width in Inches = 9, Width in Pixels = 648
Image Width in Inches = 8, Width in Pixels = 576
 
If you need to re-size your image, you can follow the steps in this tip.