Why my photos sometimes look too big on P/N

When I find a TIFF photo to post on P/N, I drag it into Mac Photo for conversion to JPEG. My TIFFS are saved at about 10" X 6.7" at 300 PPI. Converting to JPEG for upload resizes them to about 14" X 9" at 72 PPI. Way too big and way too low resolution. Re-resizing them back the original 10" X 6.7" looks a lot better and clearer here.

That's actually quite a reduction in size. It's the number of pixels that count, not the nominal size in inches or resolution in PPI. Strictly speaking it is length times resolution = pixels. There is also a reduction in file size, number of bytes, for JPEG rendering, because JPEG is a format with lossy compression.

 

When you post on Photo.net, the maximum width is 1000 pixels, or something on that order, to stay within the margins. There's no specification for height, other than a maximum size in megabytes. You can usually meet that requirement by using a lower quality JPEG in the conversion. Photoshop/Image Size allows you to set a dimension in pixels, while keeping the same aspect ratio. You can set the resolution or not, but that only changes the nominal dimensions in inches or cm. Trimming an image to a certain size in pixels automatically requires resampling.

 

Most browsers default to showing the image at 100%. That is, one pixel in the image takes one pixel in the screen.

Mac Preview has a resize function in the Tools menu, I use it all the time to prepare photos for uploading to forums.