Hi, this sounds like a daft question but i swear i'm failing at doing it.

I've created various images, and i've created them at the exact size i want them. For example, 1 of my images is 450 by 150 pixels and i want it saved in .png format. I've found 2 ways to do this.

Export it as .png file but this increases image size to about 1000x600 at a guess.

The other way is save for web and devices, but with this method it tells me original dimensions are 800x600 (which i'm sure it refers to the grey and white grid under neath). Changing these dimensions doesn't change the image to the desired dimension, but rather changes the grid.  

The only way i've found to do it is copy the image to clipboard, paste it into preview and save the file as .pdf. This results in the exact image i want with the correct dimensions and resolution etc. Except it's in .pdf so i cant use it for css image replacement. Converting or saving this image as an image extension increases its size again.

Anyone help? I want to save the images i've created 'as is'.