Usually nowadays magazines DO stipulate what form they prefer photos to be sent in. This info will appear in the magazine's writers' guidelines, which quite often nowadays are published on the web. If no information is provided, I would always ask the editor what he/she prefers. A quick phone call or e-mail should be sufficient to establish this.
One thing NOT to do is send a lot of full-size digital photos unsolicited in a huge 1MB-plus e-mail. The chances are that it will blocked by the recipient's e-mail system anyway, but if it is not the editor will simply curse you for tying up all her bandwidth. A common method nowadays with digital pix is to send low-resolution 'thumbnails' by e-mail in the first instance. If the editor likes what she sees, she may then ask you to burn the full-size photos to a CD and send it by post. Some editors do still prefer colour transparencies though.
As regards cameras, I'd say any modern digital camera with a resolution of 5 MP or better will be more than sufficient for the average article writer, unless you really intend to get into photography in a serious way.
Finally, could I just mention that my course Quick Cash Writing has an entire module devoted to photography, which goes into much more detail than I have been able to do here. More details at
www.quickcashwriting.com.
Hope this helps,
Nick GM