Everything in this video section (starting here) applies to web page images too, except that case is not about your own monitor. Digital images are dimensioned in pixels.
HTML source WIDTH and HEIGHT parameters for images must show the exact actual image size in pixels (always show the EXACT size it actually is). Triple check this, the alternatives are bad. If you should instead try to resize it with HTML, the visitor's browser will resample it after the download, but browsers are not very good at this, and it reduces image quality without reducing file download size at all, unacceptable on both counts. If the image isn't the correct size, then create one that is.
Use JPG for web photos of course, the small file size is necessary for modems. But show decent images, don't ruin them with excessive JPG compression (see JPG). Use GIF for graphics, of only 16 colors when appropriate (see GIF), or line art when appropriate (see line art) for much smaller files that have no JPG artifacts. However grayscale can appear better quality for small drawings.
Large images are slow to load. A color image at 600x400 pixels is very large for a web page. A decent quality JPG file that size will be about 72K bytes after JPG compression to 1/10 size. A 28.8K modem only moves maybe 2.7 KB/second (27 seconds for each 72 KB image file), or 56K modems maybe 4.7 KB/second (15 seconds). Smaller images are probably better for most visitors.
Some of us use 800x600 pixel screens, so it's a good idea to check your site on an 800x600 screen, and large sizes too. Screens vary in size, so never size the HTML text in pixels or points. Rule One is to always let screen text size float (no dimension) so the visitor can resize text for their situation. Any pixel dimension will be wrong in many cases. Never dimension tables or div in pixels - use percent to allow the page layout to adjust to different window sizes (except for images - the quality suffers). This is the way the HTML can work for all.