Just reply with a quote. If they say no, you haven't lost anything, if they agree to pay you are quids in. Simple.
you'd actually be very surprised at how often magazines will happily pay for the right content
starting from today- let's make a pact, no more free content, ever, and when was the last time you took notice of a photo credit?
although having tear sheets is nice...
and regarding your image, you say "they have a 72dpi image but I dont want them to have a 300dpi"
well, they already do- your 72dpi image is also a 300dpi image, and a 3000dpi image, and a 300,000dpi image (albeit the size of a pin head)
there is no 300 dpi, or 72 dpi
the DPI has no influence on the size of a digital file, DPI refers to 'inches', physical size- and physical size is determined by the pixel count, DPI is just a way of translating pixels into reality. Each printing device has a specific input requirement, i.e. the number of dots per inch it will actually lay down on the page (usually between 250 and 400).
her's some maths: e.g. if you have a 1,000 x 1,000 pixel image, that's 1,000,000 =1mp
dpi is measured as an inch long line, not as an inch square as many people believe, so
printed at 1,000dpi you have a 1" wide image (and 1"tall), make it 100dpi and you get a 10"wide image
think about it another way- if you have 100 marbles, and a 1m long ruler, and place a marble on each centimeter marking, you have 100 marbles per metre- you can lengthen the ruler to 2m and put a marble every 2cm, and now you have 50 marbles per meter, go the other way and make the ruler 50cm and now you have 200ma/m, make it 5cm and you get 2000 marbles per meter. If I told you that from a comfortable distance away you can start to see the space between the marbles at anything above 1ma/m, so going lower than that will allow you to see the ruler underneath, and going higher than that won't give any 'resolution' advantage because the human eye can't see the difference.
Printing is no different, 300dpi is the theoretical limit of perception so any higher and you won't see extra 'detail', but 200dpi is acceptable, and even as low as 150-120dpi for larger prints.
so as you can see, there is no '72dpi'
your monitor isn't even 72dpi- the real resolution is to take the pixel count, take a ruler, and then divide the width of your screen by the number of pixels wide- my macbook= 113, the new retina display macbook pro=220
what does all this mean, well- if you exported your file from lightroom as a '72dpi' file you'll notice the file size is the exact same as if you exported it at 300dpi, the only difference is that photoshop will read your 300dpi file as say 12mp, but will see your 72dpi file as something like 40mp because it's spreading out all those pixels over a much much wider area
to cut a long story short, adjust megapixel output, not dpi
/pedantic rant