First, make sure you selected "show signature." Also your post must be longer than your sig. For instance if your sig is "www.dslreports.com" your post would need to be at least 19 characters long.
It is your responsibility to find the answers to your questions. No one can just give them to you because they don't know what needs fixing. When you have searched for the answer you're looking for in the site FAQs and the various forums and still need help then it's time to ask your question.
The preferred dimension of an uploaded/linked graphic is 640 pixels or less in width.
Using the Got Attachments Feature Any graphic wider than 700 pixels uploaded via the "got attachments" feature will be auto-thumbnailed down to 200 pixels. Anything between 1 and 700 pixels wide is shown as is, unless the image file is very big in which case it is thumbnailed, and then is displayed using 1 to 3 column-spanning cells and as many rows as necessary for all attachments.
At this time, all .png graphics uploaded are reduced to 200 pixels.
The image below was uploaded as a 1600 pixel-wide .jpg file. The system auto-thumbnailed it to a smaller image clickable to see the original large version.
Using Image Tags for Linking Off-Site Any graphic wider than 700 pixels linked off-site using [img] tags will not be resized, and will blow out the right margin of the posting window. The extended margin will affect the entire posting page, and can render the thread difficult for you and others to read. If this occurs, mods will edit the graphic to include a "width" statement, changing the graphic size. Please be considerate of others when posting images.
If you must use "inline" images from another site, you can also bury width, or width and height attributes in the image html, to stop margin blowout. If you see a wide external image, "hey-mod" it, so a mod can edit in a width attribute.