In my experience, this is more a balance between load speed and photo quality. Given i'm a professional photographer myself, I understand this difficult challenge.
I feel that the internet has gone a long way and there are now some great techniques and tools that you can utilize to avoid this issues. None of these techniques are germane or specific to a photography website.
Regarding your first question: How much quality can I sacrifice against professionality?
In my experience, generally you can find a balance by playing with the settings of a jpeg image. Settings of 70-85 quality will be sufficient. Additionally, utilizing 72 DPI is sufficient to minimize file size.
Your second question: Does it indeed reflect on the photographer if the pictures are in low resolution or will people simply understand the nature of the web?
Yes, any pixelation and/or poor quality of an image will reflect poorly if the photos aren't very high quality. A photographer's work is solely dependent on visual quality and attention to visual detail.
Techniques to consider
I would avoid extreme compression if you must. The internet provides lots of great techniques to improve load:
- Provide excellent user experience: Providing your visitors rich feedback will reduce frustration and will keep them at your website longer. Consider the following UI related techniques:
- Utilizing a loader
- Interactive image zoom makes a lot of sense.
Dynamic Image Caching - Utilize website software that generates different sizes of images for the user ahead of time. If you're using a LAMP stack PHP offers two great code libraries (GD & Imagemagick and lots of great scripts that can do this for you.
Utilize a CDN: Consider architectural (server) setups. Like loading static images from a CDN or a different network altogether will provide you a quicker load time.
Utilize a image resizing Service: There's a great thread right below this one about this very topic! Smush.It or similar lossless image shrinking with API
Finally, Review YUI Performance guide: Yahoo offers an awesome guide on performance. Some of these tips are mentioned above already but this guide is pretty comprehensive.