Photoshop issue saving image as lesser filesize then reopening and saving again

Question

This is driving me absolutely mad.

I save for web at 100%. I open back up. I save again at 100%. It is bigger than the last time. It keeps doing this over and over and over it could be infinite filesize but still only exact same quality image.

What I need to do is make one sigle aspect of the image 100% resolution (my logo) but make the rest and NOT my logo a smaller filesize. What SHOULD work, is to save WITHOUT the logo, and decrease, say 70%. Then open back up the SMALLER image, add the logo and move back to 100% quality and should be SMALLER THAN THE FIRST IMAGE.

EXAMPLE:

1000KB image (1010KB with logo)

save without logo at 70% Open the now 700KB image in photoshop.

Add 10KB logo, save at 100%

IMAGE IS NOW 1310KB!!

BUT THE FIRST TIME IT WAS ONLY 1010 KB WITH LOGO AT 100% QUALITY!!

What it SHOULD be is:

710KB

Do you get it? THIS MAKES NO SENSE! I HAVE WASTED 5 HOURS ON THIS SH*T!!

WHAT THE HELL??

feel free to edit this question to remove my emotional frustration. Right now I can't think clearly I am so mad right now.

Answer 1

To understand this , you need to learn a little on compression:

When you save a file at 70%, you did a little compression. When you open the file, the file is decompress.

When you re-save at 100%, you save it in his uncompressed form.

Even uncompressed, the file size can't be calculated like that. Merge it two files for 50 Kb won't necessary equals 100Kb.

And Jpeg is already a compress file. So two images of the same size (like 1024x768) won't be the same size.

Answer 2

I don't have photoshop infront of me right now, but this tutorial on variable quality may be helpful: http://etutorials.org/Adobe/How+to+use+Photoshop+7/Part+4.+Converting+Files/Task+8+How+to+Optimize+Files+with+Variable+Compression/

Also, Fireworks is said to be good at that, but I don't know exactly how to use it. Maybe not the best help, but good luck!