A number of picture sites contain ways to resize the images built into the URLs, which makes it easy to provide thumbnails with links to the full-sized images, or just to post pictures that aren’t so big that they’ll destroy the friends list page of anyone who sees it. For instance, here’s an image I just uploaded to the LiveJournal scrapbook, along with its URL:
http://ic.pics.livejournal.com/allah_sulu/659446/795968/795968_300.jpg
See that “_300” at the end? That tells LJ to provide a smaller version of the picture (maximum resolution in either direction of 300 pixels). You can change that number to other values (100, 300, 600, 640, 900, and 1000 all seem to work; I don’t know what other numbers are legal) to display the picture in different sizes. If you want to display (or link to) the original full-sized image that you uploaded, replace the number at the end with the word “original”, like so:
http://ic.pics.livejournal.com/allah_sulu/659446/795968/795968_original.jpg
If you’re using an LJ client that automatically gives you a reduced-size version of your images, a simple search and replace operation can resize one or more images in one fell swoop. (This is how the current LJ scrapbook works; the old system from a year or two ago had a different system.) If you see a picture in someone else’s journal or community that you want to save, you can also check the URL to see if there’s a larger version available.
http://4.bp.blogspot.com/-3_6DaAJ_AAE/UXKnncJMCYI/AAAAAAAAjxo/HE05jynSQ7A/s400/Comicblocked-1.jpg
This is a picture from Blogger. The “s400” in this URL provides a thumbnail with a maximum length and/or width of 400 pixels. (Every other number that I tried worked.)
http://th00.deviantart.net/fs71/150/f/2012/332/b/8/__f_i_r_s_t__f_l_i_g_h_t_by_bluefley-d5mf8wi.jpg
This is a picture from DeviantArt. The “/150/” in this URL provides a thumbnail with a maximum length and/or width of 150 pixels. Replace the “/150/” with “/PRE/” to get a larger sized image, and remove that section of the URL entirely to get the
full-sized original.
http://24.media.tumblr.com/a6db79eb8f0d76fa58e09a0620975933/tumblr_ml0hi2CroM1qiq0dxo4_250.jpg
This is a picture from Tumblr. The “_250” in this URL provides a thumbnail with a maximum length and/or width of 250 pixels. (Legal sizes appear to be 75, 100, 250, 400, 500 and 1280.)
http://robomonkey.files.wordpress.com/2011/07/funny-graphs-pie-better1.jpg?w=450&h=180
This is a picture from WordPress. The height and/or width can be specified by adding parameters to the end of the URL; remove those parameters to display the original image.
http://img-fotki.yandex.ru/get/4117/10706545.60d/0_841d8_958d51ab_L.jpg
This is a picture hosted at a Russian site. That “_L” at the end means “LARGE”. You can change the “L” to “S” or “M” (small or medium) to reduce the size of the picture, or increase the size by changing the “L” to “XL” or “XXL” (and “orig” gives you the original size).
For all of these sites, the picture can be reduced in size, but changing the URL will never make it larger than its original size.
Note that this is entirely different from merely using the height="###" and width="###" properties of the
tag: When you do it that way, the image is being displayed in a different size, but the full-sized image is still being downloaded. Telling the site to send a smaller version of the picture reduces bandwidth and loading time.