Posting graphics for print use

We want to post graphics on our web site, suitable for magazines to 
download and publish. Questions:

1. FORMAT: JPEG, TIFF; CMYK, RGB; DPI

What format should we use? We are presently using high-quality JPEG, 
300 dpi, CMYK; typically 250K for a 4x5 inch image.

I think 300 dpi is overkill but apparently that's what at least one 
magazine asked for; not sure what is industry-accepted.

Not sure about CMYK either -- one problem is that if a user clicks on 
the JPEG, they see a very bad image as the browser doesn't recognize 
CMYK.

Maybe should use TIFF? And if so, how should we encode? LZW 
compression? I know this used to be problematic, especially 
cross-platform.

2. ENCODING: JPG, SIT, SEA, ZIP, EXE

We have the raw JPG up there and problem is most users click on the 
link. They get just the upper left corner (since it's 300 dpi) and a 
bad on-screen image (since it's CMYK). We can tell them to alt-click 
or option-click to download rather than view but few will follow the 
directions.

I'm thinking we should have them archived in Zip and StuffIt.

3. WHAT OTHERS ARE DOING

I surveyed the web and found a very low level of care given to this, 
even in press pages of vendors like IBM. Most posted a simple link to 
an RGB JPEG, about 500 pixels wide, and left it at that.

IBM had JPEG and TIFF links (not sure how they were encoded).

Apple posts BinHex encoded TIFs (can Windows users get at these??). 
Their images are Mac-TIF, LZW compressed, RGB 272 dpi.

No one used ZIP or SIT. Looks to me like everyone has skirted the 
resolution issue entirely.

Anyone have experience with this?...

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