If you are viewing in the theme’s image Attachment page, depending on theme, you may be seeing an intermediate-sized image. In order to accommodate the maximum image width for this theme, the image is being dynamically resized. In most themes clicking on the Attachment page image will bring you to the next image in the Gallery. You can get to the full size image by clicking on the link just above the image.
Images viewed in the Photo Carousel are dynamically resized to fit the browser screen on which they are being viewed. In some cases, this image may be much smaller than when viewed in an image Attachment page.
In order to preserve image quality, it is highly recommended to resize your images yourself to the maximum width for your theme in an image editing program before uploading it to your Gallery. Panos has written a highly useful post on image width on his wptips blog.
Further information about behavior of Attachment pages further down in the FAQ.
This image is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License. Attribution information on my “About” page.