The SNP's foreign affairs spokesman was not in the House of Commons to hear Prime Minister David Cameron make his argument for air strikes.
The Scottish Conservatives, Scottish Labour and the Scottish Lib Dems were all critical.
The SNP defended MP and MSP Mr Salmond saying he was on constituency business.
A spokesman stated that Mr Salmond attended a "full briefing" on Wednesday from the security services and the joint chiefs about the Syria situation and there was "nothing in the PM's statement which was of surprise or new".
He added that SNP leader in Westminster, Angus Robertson, had responded as planned to Mr Cameron's address.
And when the issue is fully debated in the Commons, Mr Salmond would make his contribution as his party's foreign affairs spokesman.
After attending First Minister's Questions at Holyrood, the former SNP leader visited the Scottish National Portrait Gallery where he unveiled the painting, by artist Gerard M Burns.
Later Mr Salmond is due to host a Scottish Parliamentary reception for veterans, serving armed forces personnel, and local community representatives.
He will also brief MSPs on the SNP position on Syria.
However, the long-arranged portrait unveiling was criticised by Ms Davidson who said: "There is nothing more serious than the potential deployment of our service personnel abroad.
"Alex Salmond has form in unveiling vanity projects to himself but he needs to remember he's paid - twice over - to be a parliamentarian, not an art critic."
Mr Rennie added: "Nothing gets in the way of Alex Salmond's ego including debating war and peace in our country and the Middle East.
"His absence from the Commons at this critical moment means we will be more sceptical about his motivations the next time he expresses outrage on the matter."
A spokesman for Scottish Labour said: "If Alex Salmond was chocolate he would eat himself. On the day that he had a job to do in parliament over Syria it's ridiculous that the SNP's foreign affairs spokesperson was in Edinburgh to look at a picture of himself."

The hosts, chasing 379 to win, began slowly and Chopra's caution increased after a middle-order collapse of three wickets for 25 runs.
The opener laboured for five and a half hours for his 17th first class century off 269 balls and ended on 119 not out.
Tim Ambrose (29) also played a patient innings, despite being forced to use a runner after suffering a leg injury.
Hampshire bowled tightly on a pitch that became easier for batting as the day went on, with Warwickshire scoring 65 runs in the morning session.
Ian Westwood (40) was trapped lbw by Andre Adams before lunch and three quick wickets after the interval strengthened the home side's decision to play for the draw.
James Tomlinson picked up two scalps, including Ireland captain William Porterfield (29), before Chopra guided his side to safety.
Despite scoring just 26 runs in the post-lunch session, Chopra scored more freely as he moved towards his first century as Warwickshire skipper.
The draw came shortly after Ambrose chipped a full delivery from Will Smith to Gareth Berg at extra-cover.
The wicket-keeper batted for an hour with a runner after suffering a gluteal strain before tea, which may rule him out of Sunday's game against champions Yorkshire.
Warwickshire captain Varun Chopra:
"They bowled well and made it hard for us to score on a pitch which got slower the longer the game went.
"At the start of the day our intention was to go for the win but it was a case of taking it session by session. After Sam Hain got out and we were four wickets down, and we didn't really have any momentum to the scoring, it was always going to be pretty tough.
"But overall I was quite pleased with the way we played. There were some good performances from us, with a six-for and two centuries."
Hampshire captain Jimmy Adams:
"At the start of the last day I thought a draw was probably the least likely result.
"The previous evening Warwickshire had got away from us a little bit and if they had started the same way today then things could have been a lot different.
"But the guys bowled well and I am really happy with the way we played over the four days."
Former first minister Alex Salmond has been criticised for unveiling his new portrait in Edinburgh on the day MPs discussed military action in Syria.
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An unbeaten century from captain Varun Chopra secured a draw for Warwickshire against Hampshire at Edgbaston.