Actually this issue involves another issue which has to be pointed out first. It is about the permissibility of chatting with members of the other sex via internet.

Regarding this, Dr. Muzammil Siddiqi, stated the following:

“Internet chat is very similar to writing letters or talking to someone on phone. Actually it is a combination of both. Muslims have to observe the same rules as they observe in writing letters or making telephone calls. Islam does not permit love letters or intimate conversations between males and females who are not married to each other.
In all our correspondence and conversations we must observe haya’ o r modesty. Boys and girls should not chat with each other just for socialization or passing time. It is haram (unlawful) for a non-mahram Muslim male and female to indulge in long conversations with each other unless it is necessary for education or for business. All conversation must be decent. The Qur’an reminds us again and again that all our words are recorded and we shall be held accountable for our words as well as our deeds on the Day of Judgment. (See Al-Ahzab 33:70; Qaf 50:18)

If one finds an interesting partner through Internet and there is a desire to know more about each other in order to get married, then one should involve one’s elders in this matter. The elders or some responsible friends should do the investigation and negotiation on one’s behalf. Even when a person wants to talk to that lady, it is good to have an elder present in this chat. In Islam the khalwah (privacy) with the non-mahram female (ajnabiyyah) is forbidden. Khalwah of course occurs when a male and a female are alone in person. But a virtual khalwah can also happen through conversation by phone or Internet. Just as men and women should not be alone with each other, in a similar way they should not be alone to talk to each other on phone or via Internet chat, especially if this conversation is about personal matters.”

Focusing on the issue of looking at the  pictures of the lady one wants to propose to, we’d like to state that there are many hadiths that urge the prospective spouse to get a look at the other before undertaking the marriage. Once a Companion told the Prophet (peace and blessings be upon him) that he was going to get married. The Prophet (peace and blessings be upon him) asked if he had seen her. When the man said ‘no’, he (peace and blessings be upon him) said: “Go and look at her for it is more likely to engender love between the two of you.” (Reported by Ahmad and others and classed as sahih or authentic)
Moreover, he (peace and blessings be upon him) said in another hadith: “If one of you proposes to a woman and if he is able to look at that which motivates him to marry her, let him do so.” (Reported by Abu Dawud & others)

Now coming to the matter of what a man is allowed to see from his prospective spouse, the acceptable view of scholars is that he is allowed but to see her face and hands or feet. There is no clear evidence to permit him to see her hair or leg. Since there is nothing to clearly permit it, there is no justification in allowing him to see her hair or legs. The general rule of prohibition stands fast unless there is a clear text, and since there is no such text, it is ruled as impermissible.