Converting TIF files to JPEG while preserving 300dpi resolution

I am creating an image in photoshop that a printer is going to use to make a postcard to advertise my website. He needs it in 300dpi resolution in order for it to print correctly. I have already created the vector image that is in 300dpi res, but need to add photos to the image. He said that everything needs to be in 300dpi resolution, including the photos that I add to the image.

I already have one photo that is in jpeg format that is at 300dpi, but I did not create that. I scanned in a photo at 300dpi as a tif file, and would like to convert it to jpeg while preserving its 300dpi resolution. I have tried everything in Photoshop that I know (which is not all that much) to try to accomplish this task, but to no avail.

ummm
hmm if its a tiff maybe the color is set at cmyk ?
I don't think you can have cmyk jpg's.. check that first :\

Why would you want to compress a 300 dpi image? A tiff is one of those formats that offers great quality for high res images. Why not just convert the jpg to tiff or keep them mixed?

Because I want to add a drop shadow to the image before I put in on the vector image, and Photoshop will not allow me to do that with a tif image.

yes it will
just change the color to rgb

It is already in rgb, and when I go to the Layer menu to try to add the drop shadow, all of the selections are grayed out. I can't even add a layer.

You can do anything to a tiff that you can do to a jpg. You can add a drop shadow to all modes except indexed. Are you trying to add a drop shadow to the background or default layer?

it should work :|

Whichever layer will work. Right now, there is only one layer that exists, the background layer. At least that is what it calls itself in the layer section.

you can't edit the background layer :P, drag it down to the new layer button then you can do whatever