Does #18 and #34 appear to have the same width on your screen Rexone?
They are identical on my screen. They have to be as they are both 800 px wide. On different screen setting, as mentioned, they will appear as different size but both still the same regardless of what that size is because both pics are still 800 pix wide.
You have to separate screen resolution from picture resolution in your mind... they are not the same animal. Screen setting (your screen) is set for viewing ie: 600 x 800, 768 x 1024 etc etc etc, a myriad of other choices with different screens. Picture resolution or more accurately for this discussion, "picture size" is a set thing when that picture is saved. If it is saved at 800 pixels wide, that is what it is until someone else actually resizes the actual photo using some form of software. So (2) 800 pixel wide pictures will appear identical width on any given screen. They will just appear smaller on a screen that is set to a higher "Screen" resolution. So that 800 pixel wide picture will fill the screen of someone who's screen size is set at 600 x 800 wide but will appear small to SP388 because his screen size is set much higher at 1680 wide x 1050 (will fill only a little under half his screen width)...
To further the discussion at the risk of confusion (hope not) I will briefly discuss actual picture resolution, in other words, dpi or (dots per inch) the picture file is actually saved at (completely different than screen resolution or screen size). Since I do both website and print work I have a good grasp of this but it was not an overnight thing and something that easily confuses many at first glance. The pictures you view online are "typically" saved at a resolution of 72 dpi or 96 dpi. The argument is that that is all the eye can see on screen. I say argument because there is one if you research this subject... but we won't go there for this basic discussion of picture resolution. Photos saved for print, by contrast, (and photos typically saved by higher end DSLR camera's) are a higher resolution. The general standard for print photos is 300 dpi. Some cameras produce this resolution directly and if not a Photo editing program like Photoshop or Fireworks can easily interpolate (resize) them. So you can have an 800 x 600 photo saved at 72 dpi and another similar 800 x 600 photo saved at 300 dpi. Both these photos appear identical dimensionally on a computer screen but the 300 dpi photo will have a hugely larger file size (more color information saved in the file). This is the reason websites don't want hi resolution OR large size (even at low resolution) photos posted as they gobble up server and user computer resources unnecessarily. Large file size photos also impact download time to the user computer regardless of where the pictures are hosted. Conversely, low rez photos don't work on a printing press. It takes much higher resolution to print sharp and clear (hence the 300 dpi standard for print work). You can plainly see this by taking a low rez photo from the web and printing it out on your high end ink jet printer... it will be soft (at best) and blurry (at worst) depending on the photo quality.
This is why ideally pictures designed for use on the web or more specifically a forum site like this one, should be about 800 pixels wide at about 72-96 dpi. Most users have their screens set at around 1024 x 768 these days and it's what "most" websites are designed to work with. Obviously as said before this size picture will appear small to someone with high screen size settings. This screen size category is not the majority though in terms of a cross section of computer users. Most have a typical 17 to 19 inch monitor (measured horizontally like a TV) these days with 1024 x 768 rez being about the average.
Hope that was not too basic or lengthly. I know there will be basic users and those with higher knowledge levels reading so I always try to lay this out at a basic level so those folks can begin to understand all this stuff.