Courtesy should dictate that a member posting a picture resize it to something respectable. This is especially helpful for members with a slow connection. But what you are talking about is extremely large pics posted. It might take your computer a few seconds or more to open them. But as you said, once open you can read the brand, size, and mounting instructions printed on the side of their tires!!!!
Xfaxman recently taught me this. When posting with the intent of adding a pic, click on "Go Advanced". Then above where you type your data, and to the far left you'll see a "blue A" block, click on that. Place your cursor where you want the pic to insert. Then in the second row of icons and 3/4 across toward the right side you'll see what appears to be a picture with a square around it, if you place your cursor over it you'll see "insert image". Click on that and then follow instructions for accessing your pic using "my computer" including "upload". Your pic will appear where your cursor was sitting. It will be in a small thumbnail size. If you want it to be larger, double click on the pic. Then select whatever size you want and it will post the pic in your post in the size you chose. Pretty slick. Here's one posted in "full size". This is a pic that has been resized prior to posting so it's manageable. Do the resizing with whatever program you have in your computer. I use "Photo Gallery".
View attachment 465298