Photoshop :: Making 100mm X 150mm Postcard

I have designed a postcard which MUST be exactly 100mm x 150mm. This is being professionally printed mail-order in a batch of 60, so I don't get any tryouts - it has to be correct when I send it.The postcard consists of one image and some text. I want it to be crisp, clear and hi-res.At first I tried to do it in Photoshop 7, but I cannot work out how to get the text next to or around the image.
 
Then I made a beautiful postcard in Indesign CS2, setting the setup page to 100 x 150. This took 2 hours and it is perfect, HOWEVER, when I export it as a JPG or TIFF it comes out really tiny on the screen and I fear that if I send it to the printer like that it will get stretched to 100 x 150 and consequently get distorted and blurred.I tried exporting it as a PDF then converting that PDF to a JPG but the colours went all blotchy.
 
So I am back trying to do it in Photoshop 7 but I cannot work out how to make the image exactly 150 x 100 and also get text around it.