Why can't I e-mail photographs in high resolution?

I'm need to send out High Resolution images from my MacBook Pro on a very regular basis. I used to use Dropbox and most ofter simply e-mail since that suits most of my clients. This has been fine until recently. Suddenly, the photographs all seemed to reduce in size when I sent them and at the moment the only way I can send High Res images is through WeTransfer. This seems to work but it's inconvenient at times and their server sometimes goes down. I need to be able to send one or two high res images quickly in an e-mail. If anyone can help me please I'd be very grateful.

thank you

I can't fine the appropriate operating system in the list below so here it is:

Software OS X 10.8.5 (12F45)